This invention relates to cathode ray tubes (CRTs) for color television and other color displays, such as data/graphic displays and more particularly relates to a magnetic ferrite ring for static convergence correction of such a CRT, and to a holder for affixing such a ring to the neck of a CRT.
CRTs for color television and related applications employ a display screen made up of a repetitive pattern of triplets of cathodoluminescent phosphor elements, each triplet including one element for each of the primary colors red, blue and green. Conventionally, the elements are in the form of closely spaced vertical stripes. The screen is activated by simultaneously sweeping three electron beams across the display screen, each beam modulated in intensity in accordance with a display signal corresponding to one of the three primary color components (red, blue, green) of the image to be displayed. In order to insure that each of the beams lands only on the phosphor elements of the corresponding color, a mask having a large number of apertures is positioned a short distance behind the screen in a manner so that there is registration between the electron beams, the apertures in the mask and the corresponding phosphor elements on the screen.
In a conventional color CRT, the number of individual picture elements or pixels, which are defined by the apertures in the mask, is typically about 250,000 or more. The large number and small size of these pixels enables the eye of the observer of the display to integrate the separate red, blue and green elements into a full color image at normal viewing distances.
Registration between the mask apertures and screen elements is critical to obtaining a high quality display image. Such registration is achieved by mounting the aperture mask behind the glass display window, and then using the mask as a photomask to form the screen in situ on the window using photolithographic techniques. The mask and screen are thus "married", and remain together throughout the remainder of the manufacturing process and operating life of the tube.
Accurate placement and alignment of the electron gun in the neck of the CRT envelope are also critical to achieving registration of the electron beams with the proper phosphor elements on the screen. One measure of such registration is the "static convergence", defined as the condition in which the three electron beams overlap in the center of the display screen, in the absence of any beam deflection current.
It will be appreciated that despite careful controls imposed at every step of the manufacturing process, some incidences of misalignments will inevitably occur. Once the manufacturing process has been completed, there is no longer any opportunity to adjust the alignment of the mechanical components inside the CRT envelope. However, the alignment of the electron beams is customarily adjusted by placing permanent magnets in proximity to the electron gun, to correct static convergence, as well as color purity and frame errors.
Such corrections have in the past been carried out manually, by adjusting the positions of a series of multipole magnetic ferrite rings with respect to the beams and to each other, resulting in significant labor costs as well as human errors.
One approach to improving such static convergence correction is to replace the manually adjustable series of ferrite rings with a single flexible ferrite tape, wound around the outside of the neck of the CRT, and custom magnetized to achieve convergence correction. A typical width for such a tape is about 2 inches.
Drawbacks to this approach include the large amount of plasticizer needed to achieve the required flexibility, necessitating a larger than desired thickness, and leading to crack formation on drying of the plasticizer. Furthermore, for a given neck diameter, as the size of the display screen is increased, the size and mass of the tape must also be increased. However, increasing the width of the tape by as little as 1/8 inch could result interference of the magnetic field from the tape interfering with the focus of the electron gun .
U.S. Pat. No. 4,220,897 describes another approach to correcting convergence errors, as well as color purity and frame errors, which involves inducing a customized magnetization pattern in a convergence correcting device such as a single metal alloy ring, located for example inside the top cup of the electron gun.
While such method represents a significant improvement over the manual adjustment of premagnetized rings on the neck of the CRT, in practice it has been found to result in variations from CRT to CRT, due to positional variations of the magnetization apparatus relative to the rings, and to variations in the gap between the nearly abutting ends of the internal magnetic rings. In addition, the internal magnetic rings are made of a CoFeVCr alloy, which although easier to magnetize than other suitable ring materials, such as ferrite, is nevertheless considerably more expensive to purchase and fabricate.
As the competition between various CRT manufacturers intensifies, it becomes increasingly important to try to minimize as much as possible such variations, and to do so at minimal cost.